The lost city in question is not in the jungles of South America, but the smog-choked outskirts of Beijing where Brendan Connal was on his latest quest in search of a hidden China.

Connal, a 35-year-old from Norfolk, England, is one of a small but intrepid network of urban explorers who are fanning out across the industrial wastelands of mainland China, uncovering the dramas and mysteries of its astonishing economic rise as they go.

“Each time I find a new site I feel excited,” says Zhao Yang, 29, another “urbexer” who documents his unconventional treks through abandoned military installations, cobweb-choked cinemas and derelict factories on a website called Cooling Plan.

Approximately 200 members make up China’s urbex community.

Worldwide, the pursuit – which some describe as “recreational trespass” – is estimated to have about 20,000 adherents, the majority in Europe and the United States.

Connal cut his teeth while living in London – for decades a hub for urbex collectives. Now based in Beijing, he has been chronicling his adventures on a blog called Burbex since late 2013.

“My instinct every time I see a wall is: ‘what’s on the other side?’” he says.

In China the answers to that question are rarely dull. Everything has an entry point. You might not be able to find it – but somebody knows the way in. Urbexing has taken him into derelict science museums , “haunted” pre-Revolution hotels, ghostly amusement parks, and a half-finished shopping centre that he calls the Great Mall of China.

Zhao, who is a professional photographer, says his edgiest find was a deactivated field hospital built to treat victims of the 2003 Sars outbreak.

Connal, whose website receives thousands of visits each month, says a growing number of western tourists are choosing to spend their holidays hiking not up the Great Wall but through the concrete carcasses of white elephant construction projects.

Sometimes, China’s urbexers bite off more than they can chew.

While touring the mortuary of an abandoned hospital in north-east China, Zhao encountered an adult’s mummified corpse and a small wooden box. “Inside, there was the body of a toddler,” he recalls, adding: “I closed the box.”

Zhao, who grew up in China’s north-eastern rust belt, says his fascination with urbex began when, aged nine, he began roaming the Mao-era bomb shelters near his home in the city of Jinzhou. “Some said the shelters were haunted by the ghosts of children,” he remembers. “We were curious kids.”

These days, Zhao lives in Beijing and his trips to vacant factories offer an escape from traffic-clogged mega-city whose population last year hit 21 million.

“When I’m at abandoned sites, I feel I’m the only person in the world. Everyone likes different things about urbex – I like this sense of solitude,” he says.

On a recent morning, Connal clambered over crumbling walls, fences and an overgrown railway track to reach the heart of Shougang, an industrial complex on Beijing’s western outskirts that was once home to 200,000 workers.

The area’s steel mills began production in 1919 – four decades before Chairman Mao’s communists took power – and Shougang went on to become one of China’s top iron and steel producers.

But production was halted in the lead-up to the 2008 Olympics as authorities fought to clean up the city’s notoriously toxic air .

Weeds have since consumed the ruins of this once bustling steel city.

“Everything is disintegrating here,” Connal says, hauling himself over a mouldering conveyor belt past a sign that reads: “Beware of the machines. No climbing.”

Each building contains artefacts – faded family photographs, notepads filled with the anonymous calligraphy of strangers

The urban explorers mapping China’s forgotten wastelands describe their mission as a race against time.

Zhao says his country’s breakneck urbanisation – which has seen hundreds of millions flock to the cities since economic reforms began in the 1980s – meant he would often return to an urbex site to find the bulldozers had moved in. “China is changing at an astonishing speed. It’s not like other countries where abandoned sites might stay there for a decade without being touched,” he says.

Demolition appears to be the likely fate of the Shougang steel complex.

As Connal snuck across the vast industrial wasteland – sprinting across deserted roads and courtyards to avoid the attention of security guards and stray dogs – he saw teams of excavators preparing to reduce one section to rubble.

Hours after his expedition began, he slipped out the complex’s western gates past billboards advertising plans to replace Shougang’s wreckage with an industrial-themed residential community called “S-Park”.

Posters showed blueprints for tree-lined boulevards and a luxury shopping district called Coking Plant Plaza.

Connal says he was disappointed that the days of his favourite Beijing haunt appeared to be numbered. But in a country as large as China there will never be a shortage of urbex destinations.

“I’ve heard there are ghost villages in Sichuan,” says Connal, with a mischievous smile.